1. Field of the Invention
The present invention relates to a colorant, particles for display device comprising the colorant, an image display medium and an image forming apparatus.
2. Description of the Related Art
Traditionally, various display techniques have been proposed as an image display medium repeatedly rewritable thereon, such as a Twisting Ball Display (bicolor saturationl particles rotational display) medium, an electrophoresis medium, a magnetophoresis medium, a thermal rewritable medium and a liquid crystal medium having a memorizing property. Although such display techniques are superb in image memorizing properties, problems have arisen insofar that it has not been possible to secure a white display like paper on a display screen only a low degree of contrast has been achieved.
In the meantime, as display technologies using a toner which technologies solve the above problem, display technologies have been reported in Japan Hardcopy'99 theses (page from 249 to 252), in which a conductive color toner and white particles are sealed between electrode substrates facing each other and charges are injected into the conductive color toner through a transfer layer, whereby the conductive color toner into which charges are injected is transferred to the display substrate side positioned opposite to a non-display substrate by the electric field applied across the substrates and the conductive color toner adheres to the inside of the display side substrate to display an image by the contrast between the conductive color toner and the white particles.
These display technologies are superior in the point that the image display medium is all constituted of solids and displays of white and black (color) can be switched one hundred percent in principle. In the above technologies, however, there is the problem that the conductive color toner which is not in contact with the charge transfer layer disposed on the inside surface of the electrode of the non-display substrate and the conductive color toner isolated from other conductive color toners are present. And these conductive color toners are not transferred by an electric field but exist at random inside of the substrate because no charge is injected, resulting in low contrast.
An image display medium comprising a pair of substrates and a group of particles that are sealed between these substrates in such a manner as to be movable between these substrates by applied electric field and contain plural types of particles for display (particles for display device) differing in color and charging characteristics is reported in the publication of Japanese Patent Application Laid-Open (JP-A) No. 2001-312225.
Also, organic pigments and inorganic pigments for coloring are used in particles for display device to colorize the image display medium.
However, these color organic pigments have the problem that they are unstable to light so that their colors are faded by light. Also, these color organic pigments have the drawbacks that a desired tinting strength is not obtained if the amount of the pigment to be added is not substantially increased with a reduction in the diameter of the particles for display device. The inorganic pigments, on the other hand, have the drawbacks that these pigments have small tinting strength and therefore, a desired tinting strength is not obtained if the amount of the pigment to be added to the particles for display device is not substantially increased. Also, fluorescent pigments are sometimes used to get a high color saturation. However, the particles diameter of the fluorescent pigment is large and it is therefore desired to develop a pigment having a small diameter to obtain a desired color tone. However, it is technically difficult to develop such a pigment.
As mentioned above, highly developed and highly color saturationtic color particles are desired as the particles for display device. Also, it is demanded of these particles have light fastness when these particles are exposed to light.